1. Field of the Invention
The present invention relates generally to integrated circuit amplifiers and, more particularly, to micro-power switch amplifiers.
2. Background Information
Many circuits need a way for sensing changes in a set of input values and generating a corresponding set of logic values. A common example is the detection of switch operation, where the opening or closing of a switch supplies control information to a micro-controller. A set of pins on the micro-processor will be connected to the switches, the pins acting as current outputs, logic inputs, or both as seen by the switches.
To provide the appropriate logic values back to the processor based upon the input information requires additional circuitry, consisting of not only the switches, but also the various peripheral circuits to convert these inputs into the corresponding logic values. This is commonly done through having a series of discrete circuit elements including some relatively low gain inverters each connected to a switch or some subset of the switches. Since the switches themselves often produce non-negligible impedance, the effects of these impedances are minimized by using large resistances, on the order of several hundred kilo-ohms, in the circuits between the switches and the inverters. The micro-processor senses the closing of the switches by scanning the switches, a process requiring both time and power from the processor as well as the appropriate code to be stored in the memory. Since it is common to use some of the processor pins to both supply input current to the switches as well as receive the logic values back for the processor, additional code must frequently be devoted to a pre-charging routine for these switches.
Although the details of different implementations vary, they all tend to suffer from one or more shortcomings. The need for a number of large value resistors is most easily met by using off chip resistors since the die size they would require cannot easily be produced on a single chip. The use of relatively low power inverters for the switch amplifiers to produce the logic values makes these circuits sensitive to the relation between the parameters in the micro-processor output circuits, in the switches, and in the amplifier circuits themselves. The assembly code needed for the scanning and pre-change routines the processor needs to run to up significant memory space in what are, for most applications, relatively small micro-controllers. Additionally, having a number of such amplifier circuits as well as the pre-changing and scanning of the switches would consume what is, for many applications, large amounts of power since these routines require the micro-controller to be continually scanning and sensing the matrix of switches over and over, using both code space and power. This restricts the utility of such circuits to applications that can provide both the needed space and current.
One particular application is for an array of push-button switches for use with a micro-controller. Such switches are commonly a set of individual push-buttons arranged in rows and columns and used to provide input to the micro-processor chip. In order to read out this input, there would have to be a set discrete amplifiers to create digital logic levels on the rows and columns of the switch array. The logic levels would then be stored in a register for the processor to read. The sort of discrete switch amplifier arrangement described above would be both a limiting factor in reducing the size of a push-button controlled processor and the major source of power consumption, both major limitations since an important application of this technology is in small, battery powered devices such as a remote control or keypad. In addition to their limiting power requirements, prior art methods need off chip components, relatively large code space for special keys can routines to pre-charge the switches, or both. As the number of functions, and the corresponding number of buttons, incorporated on such hand-held, battery-operated has increased at the same time as the contradictory aim of smaller size has grown more important, these shortcomings have become more acute.